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20 April 2001AAO Workshop10 Two vs multi-aperture interferometry: Two apertures: –Only one baseline at a time –No phase information –Simple (but not easy!) Multiple apertures: –Many baselines simultaneously –Some phase information (“closure phases”) –Complicated, but can be used for imaging

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20 April 2001AAO Workshop11 So why is it so @#!% difficult? Observed V always less than true visibility –Instrumental effects –The atmosphere One must calibrate the visibility scale by observing unresolved sources Calibrators must be “near” the target sources